Desalter operation

ABSTRACT

Improved separation of oil and water as well as suspended solids from the emulsion layer formed in a petroleum desalter is achieved by injection of demulsifier into the desalter vessel to achieve a higher localized concentration of demulsifier in the emulsion layer so as to promote improved oil/water separation from the emulsion layer. The demulsifier may be injected into the water layer or the oil layer in the region of the emulsion layer or directly into the stabilized emulsion layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/994,623, filed on Jan. 13, 2016, which claims priority toU.S. Provisional Application Ser. No. 62/104,234 filed Jan. 16, 2015,herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to crude petroleum desalting and to the desalterunit.

BACKGROUND OF THE INVENTION

Crude petroleum normally contains mineral salts that may corroderefinery units; the salt is removed from the crude oil by a processknown as “desalting”, in which hot crude oil is mixed with water and asuitable demulsifying agent to form a water-in-oil emulsion whichprovides intimate contact between the oil and water, transferring saltinto the water. The salty emulsion is then passed into a high voltageelectric field inside a closed separator vessel. The electric fieldforces water droplets to coalesce, forming larger water droplets. As thewater droplet volumes increase, they settle to the bottom of the tankunder gravitation. The desalted oil forms at the upper layer in thedesalter from where it is continuously drawn off for distillation. Thesalty water is withdrawn from the bottom of the desalter.

During operation of desalter units, a stable emulsion phase (also knownas a “rag layer”) of variable composition and thickness forms above theinterface between the oil and the separated bulk water phase at thebottom of the desalter. This interface will be referred to here as“oil/bulk-resolved water interface”. The formation of a rag layer ismostly due to stability of the oil/bulk-resolved-water interface causedby natural surfactants (e.g. asphaltenes, naphthenic acid) and/orsolids. Solids, particularly, can reside at the interface generating aphysical barrier against the immersion of water droplets into the bulkwater phase at the bottom of the desalter. The formation of the raglayer is especially problematic for crude with high amounts of naturalsurfactants and/or solids. The growth of the rag layer reduces workablevolume and may cause shorting within the electric circuit and forceunplanned and costly desalter shut down. Additionally, processing crudeswith high rag layer formation tendencies in current desalterconfigurations may cause poor desalting (salt removal) efficiency as aresult of the accumulation of solids at the bottom of the vessel, and/ora solids-stabilized rag layer leading to erratic level control andinsufficient residence time for proper water/oil separation. Formationof the rag layers has become a major desalter operating concern,generating desalter upsets, increased preheat train fouling, anddeteriorating quality of the brine effluent and disruption of theoperation of the downstream wastewater treatment facilities.

The water content of the rag layer may range from 20 to 95% water withthe balance being hydrocarbon (normally full range crude oil) and up to5 weight percent inorganic solids. Precipitated asphaltenes, waxes, andparaffins may also be found at elevated levels in the rag layer(compared to the incoming crude oil) which combine with particulates(solids), to bind the mixture together to form a complex structure thatis highly stable. Intractable emulsions of this kind comprising of oil,water and solids make adequate separation and oil recovery difficult.Often, these stable emulsions arising from the desalter are periodicallydiscarded as slop streams. This results in expensive treating orhandling procedures or pollution problems as well as the fact that crudeoil is also lost with these emulsions and slop streams.

Refinery sites which process high solids-content crudes have the mostpervasive problems with emulsion formation. Heavy crude oils andbitumens from Western Canada, which contain elevated levels of smallclay fines and other small solids, are particularly prone to forminglarge volumes of highly stable emulsions. With such feeds, growth of therag layer is more prevalent. These feeds are, however, being introducedto refineries in greater quantities despite two main disadvantagesrelated to the efficacy of desalting. First, the viscosity of thesecrudes can be quite high, so transport of water through the feed isslower than in high API gravity crude. Second, the density mismatchbetween water and oil is lower, so the gravitational energy gradient isreduced compared to higher API gravity crudes. Growth of the rag layerin the desalter requires either the amount of crude passed through thedesalter to be reduced or removal of the rag layer from the desaltingvessel for external treatment.

Attempts to mitigate the effects of rag layer formation are normallycarried out by withdrawal of the emulsion from the unit or by theaddition of chemical demulsifiers upstream of a desalter. The use ofdemulsifiers has proven to be effective in reducing emulsion stabilitybetween electrodes in a desalter, but may not be effective in reducingthe rag layer build-up which is mainly due to stability of theoil/bulk-resolved-water interface. The common practice for applicationof demulsifiers has been to add the chemical demulsifiers to the water,oil, or the emulsion before introducing the oil/water mixture to theelectric field, as shown by the following references:

U.S. Pat. No. 5,746,908 (Mitchell/Phillips Petroleum), discloses the useof steam to form an emulsion and then adding demulsifier to the mixture.

U.S. Pat. No. 7,867,382 (Droughton) discloses the use of demulsifier andmesoporous materials for reducing water-in-oil emulsion stability.

U.S. Pat. No. 7,923,418 (Becker/Baker Hughes) discloses the use ofacrylate polymer emulsion breakers for reducing stability of awater-in-oil emulsion.

U.S. Pat. No. 7,981,979 (Flatt/Nalco) discloses the use of siloxanecross-linked demulsifiers for reducing water-in-oil emulsion stability.

A shortcoming of the current practice is due, in part, to the inabilityof chemical demulsifiers to reach high enough concentrations at theoil/bulk-resolved-water interface, particularly at the beginning of thedesalter operation. Accordingly, the need persists for more effectivetechniques for mitigating the effects of rag layer formation.

SUMMARY OF THE INVENTION

We have now found that injection of the demulsifier into the desaltervessel can result in higher concentrations of the demulsifier in theemulsion layer which forms above the interface between the denser waterlayer and the supernatant oil layer. By achieving this higher, localizedconcentration of demulsifier in the region where it is needed, namely,in the emulsion layer itself, a consequent improvement in separation ofthe oil and water phases from the emulsion layer is achieved.

The demulsifier may be injected directly into the emulsion layer or ontoit with injectors located either in the water layer or the oil layer,facing in the appropriate direction, i.e. upwards from the water layerand downwards from the oil layer. When the demulsifier is injected fromeither the oil or water layers, it is preferably injected in the regionof the emulsion layer in order to secure the desired higherconcentration of demulsifier in the emulsion layer. Provision may bemade in the desalter vessel for locating the injection points atmultiple locations in the vessel, spaced either vertically orhorizontally from each other or both vertically and horizontally. Inthis way, the thickness of the rag layer may be controlled more readilywithin predetermined limits so that operation of the desalter ismaterially improved.

The desalting process entails mixing a crude oil with water and exposinga mixture of oil and water in the form of an emulsified oil/watermixture to an electric field to cause separation of the mixture into adenser water layer containing dissolved salts and a supernatant oillayer with the formation of a stabilized emulsion layer between the oillayer and the separated water layer, typically being located above theinterface between the denser water layer and the supernatant oil layer.This layer often contains emulsion-stabilizing solids which normallyinhibit separation of the oil and water into separate phases. Accordingto the present invention, demulsifier is added to the water layer or theoil layer or directly into the stabilized emulsion layer to destabilizethe emulsion so as to promote separation of the oil and water which canthen be removed as separate phases. Optionally, demulsifier may also beadded to the oil/water mixture upstream of the desalter.

A petroleum desalter unit according to the invention comprises adesalter vessel having an inlet for an oil/water mixture and electricalgrids within the vessel for imposing an electric field on the oil/watermixture in the vessel to cause separation of the mixture into a denserwater layer containing dissolved salts and a supernatant oil layer withthe formation of a stabilized emulsion layer between the oil layer andthe separated water layer; a water outlet for removing water and an oiloutlet for removing oil are also provided. Demulsifier injectors arelocated for injecting demulsifier into the vessel at in the region ofthe emulsion layer.

DRAWINGS

FIG. 1 shows a much simplified schematic of a crude petroleum desalterunit utilizing the option of direct injection of the demulsifier intothe emulsion layer or into the water layer.

FIG. 2 shows a much simplified schematic of a crude petroleum desalterunit utilizing the option of injection from the oil layer downwards intothe emulsion layer.

FIG. 3 shows a much simplified schematic of a crude petroleum desalterunit utilizing the option of injection from the oil layer at varyingangles towards the emulsion layer.

FIG. 4 shows a much simplified schematic of a crude petroleum desalterunit utilizing the option of injection downwards into the water layerusing an existing mudwash system.

FIG. 5 shows a much simplified schematic of a crude petroleum desalterunit utilizing the option of injection from the water using radialdistributors.

FIG. 6 shows a much simplified schematic of an injection gage forinjecting demulsifier at multiple locations and angles.

DETAILED DESCRIPTION

Desalting is one of the first steps in crude refining. It is done toremove salts and particulates to reduce corrosion, fouling and catalystpoisoning. In a typical desalting process, fresh water (also referred toas wash water) is mixed with oil to produce a water-in-oil emulsion,which in turn extracts salt, brine and some particulates from the oil.The salty emulsion is then sent to a desalter unit where the applicationof an electric field forces water droplets to coalesce. Largeelectrocoalesced water droplets settle under gravity and penetratethrough the oil/bulk-resolved-water interface to immerse into theresolved bulk water phase at the bottom of the desalter. The desaltedoil and the resolved bulk water are then removed at the top and thebottom of a desalter, respectively.

The wash water used to treat the crude oil may be derived from varioussources and the water itself may be, for example, recycled refinerywater, recirculated wastewater, clarified water, purified wastewater,sour water stripper bottoms, overhead condensate, boiler feed water,clarified river water or from other water sources or combinations ofwater sources. Salts in water are measured in parts per thousand byweight (ppt) and typically range from fresh water (<0.5 ppt), brackishwater (0.5-30 ppt), saline water (30-50 ppt) to brine (over 50 ppt).Although deionized water may be used to favor exchange of salt from thecrude into the aqueous solution, de-ionized water is not normallyrequired to desalt crude oil feedstocks although it may be mixed withrecirculated water from the desalter to achieve a specific ionic contentin either the water before emulsification or to achieve a specific ionicstrength in the final emulsified product. Wash water rates may bebetween approximately 5% and approximately 7% by volume of the totalcrude charge, but may be higher or lower dependent upon the crude oilsource and quality. Frequently, a variety of water sources are mixed asdetermined by cost requirements, supply, salt content of the water, saltcontent of the crude, and other factors specific to the desaltingconditions such as the size of the separator and the degree of desaltingrequired.

Conventional types of demulsifier commonly used in the processing ofcrude oil are useful in the present process although the process is notreliant on the particular selection of demulsifier. Among thedemulsifiers which may be used are those typically based on thefollowing chemistries: polyethyleneimines, polyamines, polyols,ethoxylated alcohol sulfates, long chain alcohol ethoxylates, long chainalkyl sulfate salts, e.g. sodium salts of lauryl sulfates, epoxies,di-epoxides (which may be ethoxylated and/or propoxylated). A usefulclass of polyamines comprises the succinated polyamines prepared by thesuccination of polyamines/polyamine/imines with a long chain alkylsubstituted maleic anhydride.

Challenged crudes (i.e. crude with a high amount of particulates and/ornatural emulsifiers) have been shown to produce a substantial amount ofstable emulsion layers (a.k.a. rag layer), accumulating above theinterface between the oil and resolved bulk water. The existence of arag layer is mostly due to the inability of electrocoalesced droplets tobreak the oil/bulk-resolved-water interface. The rag layer in thedesalter typically contains a high concentration of oil, residual water,suspended solids and salts which, in a typical example, might beapproximately 70% v/v water, 30% v/v oil, with 5000-8000 pounds perthousand barrels (PTB) (about 14 to 23 g/l.) solids, and 200-400 PTB(about 570 to 1100 mg/l.) salts. The aqueous phase contains salts fromthe crude oil. Crudes with high solids contents present a particularlyintractable problem since the presence of the solids, often withparticle sizes under 5 microns, may act to stabilize the emulsion andthe oil/bulk-resolved-water interface, leading to a progressive increasein the depth of the rag layer.

The present invention is especially useful in its application tochallenged crudes containing high levels of solids and it may also beapplied to benefit the desalting of high asphaltene content crudes whichalso tend to stabilize the emulsion layer and theoil/bulk-resolved-water interface in a desalter. The conventionalmitigation strategies carried out by enhancing the electrocoalescence inthe desalter by, for example, the upstream addition of chemicaldemulsifiers tend to be less than totally effective in reducing thestability of the oil/bulk-resolved-water interface. This is likely dueto the inability of the additive to fully reach theoil/bulk-resolved-water interface at the beginning of the desaltingoperation. Thinning of the oil film between electrocoalesced waterdroplets and the resolved bulk water phase is mainly due to thegravitational force. A slow rate of film thinning reduces the ability ofelectrocoalesced water droplets to immerse into the resolved bulk waterphase, causing the growth of a rag layer. The rate of film thinningstrongly depends on the particulates and the chemistry of the oil atthat interface and it may depend on physical parameters different fromthose of the electrocoalescence mechanism. The mechanism of emulsionstability within the electrodes, therefore, may not be the same as thatof the stability of the oil/bulk-resolved-water interface. This in turndemands the different additive treatment for the oil/bulk-resolved-waterinterface which is provided in the present desalting process.

To accommodate growth and movement of the emulsion layer in the vessel,the emulsifier inlet line may be provided with a manifold with inletports at different vertically spaced levels permitting the emulsifier tobe injected into the emulsion at one or more of the ports as required.The ports may be provided with manually or, more preferably, automatic,operated valves to control the injection of the demulsifier. Addition ofto demulsifier into the resolved bulk water phase and/or rag layer canalso be combined with addition of other demulsifiers upstream of thedesalter.

FIG. 1 shows a vertical cross-section of a desalter vessel in in whichinjection of the demulsifier takes place at rag layer level andoptionally into the water layer using a straight pipe design as a headersystem. The incoming crude oil feed to be desalted enters by way of line1 and is mixed with fresh wash water feed from line 2 in mixing valve 3to emulsify the water into the oil before the mixture is introduced intothe desalter vessel (5). Under the high voltage electric field inducedby means of electrode grids (4), the separation of the oil phase (6) andthe water phase (8) takes place with the emulsion phase (rag layer) (7)forming at the interface between the oil and water phases. Demulsifieris injected directly into the emulsion layer or the water phase by wayof line (9) with discharge outlets located along the length of the lineto promote the desired distribution into the emulsion or into the waterlayer in the region of the emulsion layer. When injected into the waterlayer, the injection is preferably effected not more than 20 cm from thelower level of the emulsion layer and even more preferably within 10 cmof the lower edge of the emulsion layer in order to promote the highdemulsifier concentration in or at the emulsion layer. The feed rate tothe injector line is controlled by means of valves (10) and (11).Desalted oil is withdrawn from an outlet in the upper portion of thevessel and passes to refinery processing in line 12; salty water (brine)containing salts washed out of the crude is withdrawn from an outlet atthe bottom of the vessel through line 13 and sent to waste waterrecovery.

With this improved control of the emulsion layer position and thickness,the injection line (9) may be positioned at a suitable fixed level inthe vessel but if different crude feeds generating emulsion layers ofdifferent thicknesses are liable to be encountered, a plurality ofseparately valved injection lines may be located within the vessel withcontrol of demulsifier to the line at the appropriate level for optimaldemulsification.

FIG. 2 shows a vertical cross-section of a desalter vessel in whichinjection of demulsifier takes place into the rag layer using a downwardfacing header in the oil phase. A header system (9) is positioned abovethe rag layer (7), with nozzles or slots facing downwards to deliverdemulsifier chemical above the interface from the emulsion/oil phase(6).

FIG. 3 shows a horizontal cross-section of a desalter vessel in whichinjection of demulsifier into desalter using injection quills varyinglocation, angle, and velocity. Injection quills (9) positioned aroundthe perimeter of the desalting vessel can be used to deliver demulsifierchemical into the rag layer (7) or onto it from above or below from avariety of angles with varying velocities and flowrates controlled bysuitable valving and control devices. Depending upon the locations ofthe mixture inlet and the oil and water outlets, and the general flowdirection of the oil, water and emulsion layers in the vessel, theinjectors may be angled concurrent with relative to the general flowaxis so as to promote flow or to promote mixing by countercurrentinjection.

Combinations of injector configurations may be used, for example,injection into bother the water and oil layers by use of an injectionline below the emulsion layer and above it in the oil layer.

FIG. 4 shows a vertical cross-section of a desalter vessel in whichinjection of demulsifier into desalter is effected using the currentmudwash system. The mudwash system (9) which is important for theremoval of solids that accumulate on the bottom of the desalting vessel.A header, located approximately 18 inches (46 cm) from the bottom of thevessel and running the length of the desalter, has a number of downwardfacing water spray nozzles designed to disturb solids and preventaccumulation on the vessel bottom. This system can be used to deliverthe demulsifier chemical to the water phase (8) within the desaltingvessel with limited modification to the existing unit.

FIG. 5 shows a vertical cross-section of a desalter vessel in whichinjection of demulsifier into the rag layer is effected using a radialdistributor design. Radial distributors (8) fed with demulsifier frommanifold (9) can be used to deliver demulsifier chemical to the raglayer (7) with a minimum vertical component of velocity. Thedistributors may simply have a flat plate over the vertically orientedoutlet conduit or have a domed or downturned cap similar to the “bubblecap” of a distillation column.

FIG. 6 is an isometric schematic of an injection system in whichdemulsifier is injected both above and below the rag layer using a cagedistributor. A cage distributor can be positioned with the rag layer“inside the cage, allowing for delivery of the demulsifier chemical fromboth above and below the rag layer. The cage injection system itself mayhave multiple lines extending above and below the emulsion layer andthese may inject demulsifier into various angles at or into the emulsionlayer. Using a cage-type injector, the demulsifier can be directedupwards, downwards as well as in any angular direction in or around thecage.

What is claimed is:
 1. A petroleum desalter unit having a desaltervessel having: an inlet for an oil/water mixture, electrical gridswithin the vessel for imposing an electric field on the oil/watermixture in the vessel to cause separation of the mixture into a denserwater layer containing dissolved salts and a supernatant oil layer withthe formation of a stabilized emulsion layer between the oil layer andthe separated water layer, a water outlet for removing water from thedenser water layer, an oil outlet for removing oil from the supernatantoil layer demulsifier injectors for injecting demulsifier into thevessel in the region of the emulsion layer.
 2. A petroleum desalter unitaccording to claim 1 which comprises demulsifier injectors for injectingdemulsifier directly into the emulsion layer.
 3. A petroleum desalterunit according to claim 1 which comprises demulsifier injectors forinjecting demulsifier directly into the denser water layer.
 4. Apetroleum desalter unit according to claim 1 which comprises demulsifierinjectors for injecting demulsifier directly into the denser water layertowards the emulsion layer at a distance of not more than 20 cm from theinterface between the oil and water layers.
 5. A petroleum desalter unitaccording to claim 1 which comprises demulsifier injectors for injectingdemulsifier directly into the supernatant oil layer.
 6. A petroleumdesalter unit according to claim 1 which comprises demulsifier injectorsfor injecting demulsifier directly into the supernatant oil layertowards the emulsion layer at a distance of not more than 20 cm from theinterface between the oil and water layers.
 7. A petroleum desalter unitaccording to claim 1 in which the oil and water layers flow from theregion of the inlet to the respective oil and water outlets in a generalflow direction and the vessel comprises demulsifier injectors forangularly injecting demulsifier towards the general flow direction.
 8. Apetroleum desalter unit according to claim 3 which comprises demulsifierinjectors for injecting demulsifier directed towards the bottom of thevessel.
 9. A petroleum desalter unit according to claim 8 in which theinjectors for injecting demulsifier directed towards the bottom of thevessel are located not more than 46 cm from the bottom of the vessel.